Hybrid gas turbine engine starter-generator

ABSTRACT

A rotating electrical machine, such as an aircraft starter-generator, that may be operated in either a DC motor mode or an AC generator mode. The machine includes a conventionally wound main stator that is selectively configurable as a multi-pole AC stator and a multi-pole DC stator. The machine also includes rotor windings that are configured to be selectively coupled to either an exciter or a plurality of commutator segments, and DC brushes that are selectively moveable into, and out of, electrical contact with the commutator segments, to thereby electrically couple and decouple a DC power source to and from, respectively, the rotor windings.

TECHNICAL FIELD

The present invention relates to rotating electrical machines such asstarter-generators for gas turbine engines and, more particularly, to ahybrid starter-generator that is selectively convertible betweenoperation as an AC generator and a DC motor.

BACKGROUND

An aircraft may include various types of rotating electrical machinessuch as, for example, generators, motors, and motor/generators.Motor/generators are used as starter-generators in some aircraft, sincethis type of rotating electrical machine may be operated in both a motormode and a generator mode. A starter-generator may be used to start theengines or auxiliary power unit (APU) of an aircraft when operating as amotor, and to supply electrical power to the aircraft power distributionsystem when operating as a generator. Thus, when operating as a motor, astarter-generator may be designed to supply mechanical output torquesufficient to start the engines.

One particular type of aircraft starter-generator includes threeseparate brushless generators, namely, a permanent magnet generator(PMG), an exciter generator, and a main motor/generator. The PMGincludes permanent magnets on its rotor. When the PMG rotor rotates, ACcurrents are induced in stator windings of the PMG. These AC currentsare typically fed to a regulator or a control device, which in turnoutputs a DC current if the starter-generator is operating in agenerator mode. Conversely, if the starter-generator is operating in amotor mode, the control device supplies AC power.

If the starter-generator is operating in the generator mode, DC currentfrom the regulator or control device is supplied to stator windings ofthe exciter. As the exciter rotor rotates, three phases of AC currentare typically induced in the exciter rotor windings. Rectifier circuitsthat rotate with the exciter rotor rectify this three-phase AC current,and the resulting DC currents are provided to the rotor windings of themain motor/generator. Finally, as the main motor/generator rotorrotates, three phases of AC current are typically induced in the mainmotor/generator stator, and this three-phase AC output can then beprovided to a load.

If the starter-generator is operating in the motor mode, AC power fromthe control device is supplied to the exciter stator. This AC powerinduces, via a transformer effect, an electromagnetic field in theexciter armature, whether the exciter rotor is stationary or rotating.The AC currents produced by this induced field are rectified by therectifier circuits and supplied to the main motor/generator rotor, whichproduces a DC field in the rotor. Variable frequency AC power issupplied from the control device to the main motor/generator stator.This AC power produces a rotating magnetic field in the main stator,which causes the main rotor to rotate and supply mechanical outputpower.

The above-described starter-generator may include relatively complex andheavy power electronics circuits in the control device. For example,some control devices may include inverters, for converting DC to ACpower, rectifiers, for converting AC power to DC power, and potentiallycomplex voltage and frequency control circuits. Although brush-type DCmachines may alleviate the need for some of these complex and heavyelectronic circuits, these also suffer certain drawbacks. For example,the brushes tend to wear fairly quickly, reducing machine reliability,and increasing the need for periodic maintenance and cleaning.

One prior approach to addressing the above-mentioned drawbacks wasdeveloped by some of the inventors of the present invention. The priorapproach, disclosed in U.S. patent application Ser. No. 10/______,entitled “Gas Turbine Engine Starter Generator with AC Generator and DCMotor Modes,” and assigned to the assignee of the instant application,provides a specially wound main stator that can be selectivelyconfigured as either a multi-pole AC stator or a multi-pole DC stator.Although this prior approach addresses the noted drawbacks, it toopresents certain drawbacks. In particular, the specially wound mainstator, which includes additional segments and windings, is more complexthan a conventionally wound stator, which can increase overall costs,and adversely affects power quality when operating in the generatormode.

Hence, there is a need for a starter-generator that does not rely onrelatively complex and heavy inverters and frequency control circuitsfor proper operation, and/or does not suffer reduced reliability frombrush wear, and/or the need for potentially frequent maintenance andcleaning and/or does not use a specially wound stator. The presentinvention addresses one or more of these needs.

BRIEF SUMMARY

The present invention provides a starter-generator that does notincorporate relatively complex power conversion and frequency controlcircuits, which reduces the weight and cost as compared to some currentstarter-generators, that may increase the wear life of the DC brushes,which reduces the need for cleaning and maintenance, and that does notrely on a specially wound stator.

In one embodiment, and by way of example only, a gas turbine enginestarter-generator includes a housing, a main rotor, and a main stator.The main rotor is rotationally mounted within the housing. The mainstator is mounted within the housing and at least partially surrounds atleast a portion of the main rotor. The main stator is selectivelyconfigurable as an N-pole DC stator or an M-pole, multi-phase AC stator.When the main stator is configured as the M-pole, multi-phase AC stator,each phase consists of two or more stator winding circuits electricallycoupled in parallel with one another, and when the main stator isconfigured as the N-pole DC stator, the stator winding circuitsassociated with each phase are directly connected in series with oneanother.

In another exemplary embodiment, a gas turbine engine starter-generatorincludes a housing, a main rotor, a main stator, two or more brushes,and one or more stator switches. The main rotor is rotationally mountedwithin the housing. The main stator is mounted within the housing and atleast partially surrounds at least a portion of the main rotor. The mainstator is selectively configurable as an M-pole AC stator and an N-poleDC stator, where M and N are each integers greater than one. The brushesare adapted to electrically couple to a DC power source and areselectively movable into, and out of, electrical contact with at least aportion of the main rotor. The stator switches are configured toselectively electrically couple the main stator to the DC power sourceand to at least two of the brushes.

In yet another exemplary embodiment, a gas turbine enginestarter-generator includes a housing, a main rotor, a main stator, twoor more brushes, one or more stator switches, and one or more rotorswitches. The main rotor is rotationally mounted within the housing. Themain stator is mounted within the housing and at least partiallysurrounds at least a portion of the main rotor. The main stator isselectively configurable as an M-pole AC stator and an N-pole DC stator,where M and N are each integers greater than one. The brushes areadapted to electrically couple to a DC power source and are selectivelymovable into, and out of, electrical contact with at least a portion ofthe main rotor. The stator switches are configured to selectivelyelectrically couple the main stator to the DC power source and to atleast two of the brushes. The rotor switches are configured toselectively electrically couple the main rotor to the DC power source.

In still a further exemplary embodiment, a stator includes a main statorbody, and a plurality of stator coils. The stator coils are wound aroundat least a portion of the main body in a configuration that allows thestator to be selectively configured as an M-pole AC stator or an N-poleDC stator, where M and N are each integers greater than one. When thestator is configured as the M-pole AC stator, each phase consists of twoor more stator winding circuits electrically coupled in parallel withone another, and when the stator is configured as the N-pole DC stator,the stator winding circuits associated with each phase are directlyconnected in series with one another.

In yet a further exemplary embodiment, a rotor includes a shaft, aplurality of poles extending radially from the shaft, and a plurality ofrotor windings wound around the plurality of poles in a configurationthat allows the rotor windings to be electrically coupled to one of twoDC power sources.

Other independent features and advantages of the preferredstarter-generator will become apparent from the following detaileddescription, taken in conjunction with the accompanying drawings whichillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic block diagram of an exemplary highspeed starter-generator system according to an embodiment of theinvention;

FIG. 2 is a perspective view of a physical embodiment of thestarter-generator system depicted in FIG. 1;

FIG. 3 is simplified representation of a main stator that may be used inthe starter-generator of FIGS. 1 and 2, which schematically depictsvarious switched interconnections between stator winding segmentsaccording to an exemplary embodiment of the present invention;

FIG. 4 is simplified representation of a main rotor that may be used inthe starter-generator of FIGS. 1 and 2, which schematically depictsvarious switched interconnections between rotor windings according to anexemplary embodiment of the present invention;

FIG. 5 is a simplified representation of the main stator, similar tothat shown in FIG. 3, with the stator windings electrically connected sothat the main stator is configured as a multi-pole AC generator stator;

FIG. 6 is a schematic diagram of the main stator windings whenconfigured as a multi-pole AC generator stator;

FIG. 7 is a simplified representation of the main rotor, similar to thatshown in FIG. 4, with the rotor windings electrically connected so thatthe main rotor is configured as a multi-pole AC generator rotor;

FIG. 8 is a simplified representation of the main stator, similar tothat shown in FIG. 3, with the stator windings electrically connected sothat the main stator is configured as a multi-pole DC motor stator;

FIG. 9 is a schematic diagram of the main stator windings whenconfigured as a multi-pole DC motor stator; and

FIGS. 10–12 are simplified representations of the main rotor, similar tothat shown in FIG. 4, with the rotor windings electrically connected sothat the main rotor is configured as a multi-pole DC motor rotor.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The following detailed description of the invention is merely exemplaryin nature and is not intended to limit the invention or the applicationand uses of the invention. Furthermore, there is no intention to bebound by any theory presented in the preceding background of theinvention or the following detailed description of the invention.

A functional schematic block diagram of one embodiment of a high speedmotor/generator system 100 is shown in FIG. 1. This exemplarymotor/generator system 100 includes an exciter 110, a mainmotor/generator 120, a motor/generator control unit 130, one or morerectifier assemblies 140, and one or more pairs of brushes 150. It isnoted that the motor/generator system 100 may be used as astarter-generator, operable at various speeds, for a gas turbine enginein aircraft, space, marine, land, or other vehicle-related applicationswhere gas turbine engines are used. For aircraft applications, gasturbine engines are used for propulsion (e.g., the aircraft's mainengines) and/or for power (e.g., the auxiliary power unit (APU)).

When the motor/generator system 100 is operating in a generator mode, astator 124 of the main motor/generator 120, as will be described morefully below, is configured as a multi-pole AC stator, and the brushes150 are preferably moved out of physical contact with a mainmotor/generator rotor 122. The motor/generator control unit 130, whichis coupled to receive power from an input supply source 105, suppliescontrollable DC power to a stator 112 of the exciter 110, but isconfigured so that DC power is not supplied to the main stator 124. Aprime mover 170 such as, for example, a gas turbine engine, rotates botha rotor 114 of the exciter 110 and the main motor/generator rotor 122.As the exciter rotor 114 rotates, it generates and supplies AC power tothe rectifier assemblies 140. The output from the rectifier assemblies140 is DC power and is supplied to rotor windings 126 wound on the mainmotor/generator rotor 122. As a result, AC power is output from statorwindings 128 wound on the main motor/generator stator 124. Though notdepicted in FIG. 1, it will be appreciated that DC power can be obtainedfrom the AC power output from the motor/generator system 100, if sodesired, by including one or more rectifiers.

During its operation in the generator mode, the motor/generator system100 is capable of supplying output power at a variety of frequencies.Alternatively, a gearing system may be used to operate themotor/generator at a constant speed and, thus, supply a constantfrequency. The output power from the main motor/generator stator 124 istypically three-phase AC power. One or more stator output leads 125supplies the generated AC power to external systems and equipment viaone or more terminal assemblies 155. The motor/generator control unit130 can regulate the power output based upon monitoring signals providedto it from monitoring devices 195. In the depicted embodiment, theexciter 110 and the main motor/generator 120 both rotate along a singleaxis 198 at the same rotational speed. It will be appreciated, however,that in other embodiments the exciter 110 may rotate along a differentaxis. Moreover, the relative positioning of the exciter 110 and the mainmotor/generator 120 can be modified in different embodiments such thatthe exciter 110 is physically located on the other side of the mainmotor/generator 120.

When the motor/generator system 100 is operating in a motor mode, themain motor/generator stator 124 is configured as a multi-pole DC stator,the brushes 150 are moved into physical contact with the mainmotor/generator rotor 122, and the main motor/generator rotor 122 iselectrically disconnected from the rectifier assemblies 140. A DC powersource 180, which is electrically coupled to the brushes 150 (thisconnection is not shown in FIG. 1), supplies DC power to the mainmotor/generator rotor windings 126, via a commutator 129. The commutator129, as is generally known, and as will be described more fully below,alternates the polarity of opposing rotor poles. The control unit 130 isadditionally configured to supply DC power to the main motor/generatorstator windings 128, and no longer supply the controllable DC power tothe exciter stator 112. It should be appreciated that the DC power thatis supplied to the main motor/generator stator windings 128 ispreferably the same DC power source 180 that supplies the brushes 150,though it will be appreciated that it could be a separate DC powersource. In any case, as a result of this configuration, the mainmotor/generator rotor 122 is rotated, supplying rotational power to, forexample, the gas turbine engine 170.

In the depicted embodiment, the brushes 150 are moved in to, and out of,contact with the main motor/generator rotor 122 using one or moreactuators 182 such as, for example, one or more solenoids. The actuators182 are controlled using, for example, brush control logic 184. In thedepicted embodiment, the brush control logic 184 is located in thecontrol unit 130, though it will be appreciated that it could be locatedelsewhere. A perspective view of an exemplary physical embodiment of atleast those portions of the motor/generator system 100 that are mountedwithin a housing 200 is illustrated in FIG. 2.

Turning now to FIG. 3, a simplified representation of an exemplaryembodiment of the main motor/generator stator 124, and portions of themain motor/generator rotor 122, schematically depicting various switchedinterconnections between stator winding segments and portions of therotor 122 is shown. It will be appreciated that the stator 124 istypically cylindrical in shape; however, for clarity and ease ofexplanation, it is shown in a flat, linear configuration. The stator 124includes a main body (or core) 302, around which the stator windings 128are wound. The stator core 302 is formed by a plurality of statorlamination sections A1, B1, C1, A2, B2, C2, each of which includes oneor more slots (not illustrated). The stator windings 128 are woundaround the stator core 302 by inserting a portion of each winding into,and through, the slots in each stator sections A1, B1, C1, A2, B2, C2,thereby forming six stator winding circuits A1′, B1′, C1′, A2′, B2′,C2′. The stator winding circuits A1′, B1′, C1′, A2′, B2′, C2′ are thenelectrically coupled, as described more fully below, to generate desiredmagnetic field polarities when current flows through the stator windings128. It is noted that, for clarity, each winding circuit A1′, B1′, C1′,A2′, B2′, C2′ is represented using only a single winding 128 insertedthrough each stator section A1, B1, C1, A2, B2, C2. However, it will beappreciated that more than one stator winding 128 may be insertedthrough each stator section A1, B1, C1, A2, B2, C2, and electricallycoupled together to form each of the winding circuits A1′, B1′, C1′,A2′, B2′, C2′.

At least two conductor lead 308 a–l extend from each of the statorsections A1, B1, C1, A2, B2, C2, for a total of twelve leads. Each lead308 a–l is electrically coupled to each of the stator windings 128 thatextend through the respective stator section A1, B1, C1, A2, B2, C2 fromwhich each lead 308 a–l extends. Three of the leads 308 a, 308 c, 308 eare selectively electrically coupled to one of three terminals T1, T2,T3, which are in turn electrically coupled to the above-referencedoutput leads 125 (not shown in FIG. 3). A plurality of controllablestator switches 310–318 are electrically coupled to selectivelyinterconnect various ones of the leads 308 a–l, and to selectivelycouple two of the leads 308 a and 308 k to the DC power source 180. Inthe depicted embodiment, the stator switches 310–318 each have at leasttwo positions, a first position (1) and a second position (2). In FIG.3, however, the stator switches 310–318 are each shown in a transitionstate between the first and second positions. It will be appreciatedthat the stator switches 310–318 may be physically separate switches ordifferent poles of a single switch. In the depicted embodiment, thestator switches 310–318 are each remotely controlled by switch controllogic 186, which may be located in the control unit 130. However, itwill additionally be appreciated that the switch control logic 186 maybe located elsewhere. It should further be appreciated that the statorswitches 310–318 may be any one of numerous controllable switch typesincluding, but not limited to, mechanical switches, relays, and varioustypes of transistors. Moreover, it should be appreciated that the statorswitches 310–318 may be physically located within the controller 130 orexternal thereto, as shown in FIG. 3.

With continued reference to FIG. 3, it is seen that the DC power source180 is also electrically coupled to the DC brushes 150. As waspreviously noted, the DC brushes 150 are selectively moved in to, andout of, contact with the commutator 129 via one or more brush actuators182 (not shown in FIG. 3), which are controlled by the brush controllogic 184 (also not shown in FIG. 3). As will be described in moredetail further below, when the motor/generator system 100 is configuredto operate in the generator mode, the brush actuators 182 move the DCbrushes 150 out of contact with the commutator 129. Conversely, when themotor/generator system 100 is configured to operate in the motor mode,the brush actuators 182 move the DC brushes 150 into contact with thecommutator 129.

With reference now to FIG. 4, a simplified representation of anexemplary embodiment of the main motor/generator rotor 122,schematically depicting various switched interconnections between rotorwindings and the rectifier assemblies 140 is shown. The rotor 122, asmay be appreciated, typically has a substantially cylindrical overallspace envelope. However, as with the stator 124 configuration describedabove, for clarity and ease of explanation, the rotor 122 is shown in aflat, linear configuration. The rotor 122 includes a main shaft 402, anda plurality of poles 404 (e.g., 404-1, 404-2, 404-3, 404-4) that extendradially therefrom. The rotor windings 126 are wound around the rotorpoles 404, and are electrically coupled to receive DC power from eitherthe exciter 110, via the rectifier assemblies 140, or the DC powersource 180, via the commutator 129. The rotor windings 126, as will bedescribed more fully further below, are electrically coupled to generatedesired magnetic field polarities in the rotor poles 404, depending onwhether the main motor/generator 120 is configured to operate in themotor or generator mode.

As with the stator 124, the rotor 122 also includes a plurality ofcontrollable rotor switches 406, 408, 410, 412 to selectivelyelectrically interconnect the rotor windings 126 in a desiredconfiguration, and to couple to rotor windings 126 to either therectifier assemblies 140 or the commutator 129. In the depictedembodiment, the rotor switches 406–412, similar to the stator switches310–318, each have at least two positions, a first position (1) and asecond position (2). In FIG. 4, however, the rotor switches 406–412 areeach shown in a transition state between the first and second positions.It will be appreciated that the rotor switches 406–412 may be physicallyseparate switches or different poles of a single switch. In the depictedembodiment, the switches 406–412 are each remotely controlled by thesame switch control logic 186 that controls the stator switches 310–318.However, it will additionally be appreciated that the rotor switches406–412 may be controlled by separate switch control logic (notdepicted). It should further be appreciated that the rotor switches406–412 may be any one of numerous controllable switch types including,but not limited to, mechanical switches, relays, and various types oftransistors. Moreover, it should be appreciated that the rotor switches406–412 may be physically located within the controller 130 or externalthereto, as shown in FIG. 4.

With the above-described electrical interconnection schemes, thedepicted main motor/generator 120 may be selectively configured aseither a 4-pole AC generator or a 2-pole DC motor. The specificelectrical interconnections for these two different configurations willnow be described. Before doing so, however, it is to be appreciated thatthe stator and rotor structure and associated electrical interconnectionschemes depicted and described are merely exemplary of one that may beused to provide a hybrid 4-pole AC/2-pole DC generator/motorcombination, and that the stator and rotor structures and electricalinterconnection schemes can be modified to provide any one of numeroushybrid M-pole AC/N-pole DC generator/motor combinations.

Referring now to FIGS. 5–7, the specific electrical interconnectionswhen the main motor/generator 120 is configured to operate in thegenerator mode will first be described. In the generator mode, each ofthe stator switches 310–318, and each of the rotor switches 406–412, ismoved to the first position (1). In addition, the brush actuators 182move the brushes 150 out of contact with the commutator 129. As is shownin FIGS. 5 and 6, when each of the stator switches 310–318 (for clarity,not shown in FIG. 5) is moved to the first position (1), the stator 124is configured as a 4-pole AC stator. Specifically, as is shown mostclearly in FIG. 6, the stator windings 128 are electrically coupledtogether in a 3-phase, wye (3{acute over (Ø)}-Y) configuration, suchthat each phase includes two stator winding circuits electricallycoupled in parallel. More specifically, one phase includes statorwinding circuits A1′ and A2′ electrically coupled in parallel, a secondphase includes winding circuits B1′ and B2′ electrically coupled inparallel, and a third includes winding circuits C1′ and C2′ electricallycoupled in parallel. In addition, stator switches 310 and 318 arepositioned such that the DC power source 180 is not connected to thestator 124.

Turning to FIG. 7, when each of the rotor switches 406–412 is moved tothe first position (1), the rotor 122 is configured as a conventional ACgenerator rotor. Specifically, rotor switches 406 and 412 are positionedsuch that the rotor 122 is electrically connected to the rectifierassemblies 140 and is electrically disconnected from the DC power source180. Thus, in the generator mode, excitation current for the rotorwindings 126 is supplied from the exciter 110. Moreover, rotor switches408 and 410 are positioned such that the rotor windings 126 wound aroundthe first 404-1 and third 404-3 rotor poles are electrically coupled inseries with one another, and are electrically coupled in parallel withthe windings 126 that are wound around the second 404-2 and fourth 404-4rotor poles, which are also electrically coupled in series with oneanother.

Thus, as FIG. 7 additionally shows, in the generator mode the DC currentsupplied to the rotor windings 126, which is shown using current flowarrows 702, induces two magnetic pole pairs in the rotor 122. The first404-1 and second 404-2 poles constitute one pole pair, and the third404-3 and fourth 404-4 poles constitute the second pole pair. As therotor 122 rotates, AC current is induced in the stator windings 128(shown schematically in FIG. 7). As shown in FIG. 5, the induced ACcurrent in turn induces two magnetic pole pairs in the stator 124. Theinduced AC currents may also be supplied to a load, via the output leads125, and may also, as was previously mentioned, be rectified andsupplied as DC current to one or more loads.

Turning to FIGS. 8–12, the specific electrical interconnections when themain motor/generator 120 is configured to operate in the motor mode willnow be described. In the motor mode, each of the stator switches310–318, and each of the rotor switches 406–412, is moved to the secondposition (2). In addition, the brush actuators 182 move the brushes 150into contact with the commutator 129. In FIG. 8, it is seen that wheneach of the stator switches 310–318 (for clarity, not shown in FIG. 8)is moved to the second position (2), the stator 124 is configured as a2-pole DC stator. Specifically, the stator windings 128 are electricallycoupled in series with one another and with the DC power source 180.More specifically, the stator windings 128 are electrically coupled suchthat the stator winding circuits associated with each phase areelectrically connected in series. In particular, as shown most clearlyin FIG. 9, the winding circuits A1′ and A2′ are directly connected inseries, winding circuits B1′ and B2′ are directly connected in series,and winding circuits C1′ and C2′ are directly connected in series.

When the rotor switches 406–412 are moved to the second position (2), asshown in FIG. 10, the rotor windings 126 are electrically disconnectedfrom rectifier assemblies 140, and the DC power source 180 iselectrically coupled to the brushes 150. As was noted above, the brushes150 are in contact with the commutator 129, which is in turnelectrically coupled to the rotor windings 126. The rotor switches 408and 410 are positioned such that the rotor windings 126 wound around thefirst 404-1 and third 404-3 poles are electrically connected in parallelwith one another and configured such that when current flows through therespective windings 126 wound thereon a single magnetic pole pair isinduced in the first 404-1 and third 404-3 poles. Similarly, the rotorwindings 126 wound around the second 404-2 and fourth 404-4 rotor polesare electrically connected in parallel with one another such that whencurrent flows through the respective windings 126 wound thereon a singlemagnetic pole pair is induced in the second 404-2 and fourth 404-4poles.

With the rotor 122 and stator 124 configured as described above, DCcurrent from the DC power source 180 flows through the stator windings128 and, as shown in FIG. 8, induces a single magnetic pole pair in thestator 124. Thus, when commutated current flows through the rotorwindings 126, the rotor 122 will be rotated, as shown in FIGS. 10–12using rotation arrow 1002, and supply rotational power to the gasturbine engine 170 or other mechanical load. In FIGS. 10–12 the depictedrotor circuit corresponds to three different contact positions betweenthe brushes 150 and the commutator 129 during rotor 122 rotation. Itwill be appreciated that although FIG. 11 shows all of the rotorwindings 126 simultaneously excited, the rotor windings 126 stillgenerate the desired pole pair flux pattern for the motor mode, incontrast to the pole pair flux pattern generated by the simultaneouslyexcited rotor windings 126, as shown in FIG. 7, for the generate mode.

Typically, when the motor/generator system 100 is being implemented asan aircraft starter-generator, the aircraft is on the ground and thestarter-generator is initially operated in the motor mode. Thus, thestator switches 310–318 and the rotor switches 406–412 are all moved tothe second position (2), and the brushes 150 are moved into contact withthe main rotor 122. As a result, the main stator 124 is configured as a2-pole DC stator, the rectifier assemblies 140 are electricallydisconnected from the rotor windings 126, and the DC power source 180supplies DC excitation power to the stator windings 128 and to the rotorwindings 126 (via the brushes 150 and the commutator 129). The fluxinteraction between the rotor windings 126 and the stator windings 128,and the commutation provided by the DC brushes 150 and commutator 129,gives rise to rotor 122 rotation.

When the rotational speed of the rotor 122 reaches a predeterminedmagnitude and is increasing, the motor/generator system 100 switches tooperation in the generator mode. Hence, the switch control logic 186automatically moves the stator switches 310–318 and the rotor switches406–412 to the first position (1), and the brush control logic 184causes the actuators 182 to move the brushes 150 out of contact with thecommutator 129. As a result, the main stator 124 is configured as a4-pole AC stator, the rectifier assemblies 140 are electrically coupledto the rotor windings 126, and the AC power output from the exciterstator 112 is rectified by the rectifiers 140 and supplied to the rotorwindings 126.

It will be appreciated that the predetermined rotational speed at whichmotor/generator operation switches from the motor mode to the generatemode may vary, depending on the type of engine that is being started.The predetermined rotational speed is preferably based on thetorque-speed profile of the particular prime mover 170 that is used, sothat the motor/generator system 100 can be switched to operation as agenerator when the prime mover 170 can supply sufficient torque torotate the rotor 122. It will additionally be appreciated that this isonly exemplary of a particular preferred embodiment, and that themotor/generator 170 could also be switched based on other operationalneeds, such as, for example, a specified time period after it beginsoperating in motor mode.

A main motor/generator 120 that is selectively configurable as amulti-pole (e.g., M-pole) DC motor and a multi-pole (e.g., N-pole) ACgenerator provides additional flexibility over presently knownmotor/generators. For example, with a selectively configurable mainmotor/generator, the motor/generator system 100 need not includerelatively complex power conversion and frequency control circuits, andcan increase time between maintenance of the DC brushes. The selectivelyconfigurable main motor/generator disclosed herein also does not rely ona specially wound stator to implement the hybrid functionality, whichcan also reduce overall costs.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt to a particularsituation or material to the teachings of the invention withoutdeparting from the essential scope thereof. Therefore, it is intendedthat the invention not be limited to the particular embodiment disclosedas the best mode contemplated for carrying out this invention, but thatthe invention will include all embodiments falling within the scope ofthe appended claims.

1. A gas turbine engine starter-generator, comprising: a housing; a mainrotor rotationally mounted within the housing; and a main stator mountedwithin the housing and at least partially surrounding at least a portionof the main rotor, the main stator selectively configurable as an M-poleAC stator or an N-pole DC stator, M and N each integers greater thanone, wherein: when the main stator is configured as the M-pole,multi-phase AC stator, each phase consists of two or more stator windingcircuits electrically coupled in parallel with one another, and when themain stator is configured as the N-pole DC stator, the stator windingcircuits associated with each phase are directly connected in serieswith one another.
 2. The starter-generator of claim 1, furthercomprising: a control circuit electrically coupled to at least the mainstator and operable to selectively configure the main stator as theM-pole AC stator or the N-pole DC stator.
 3. The starter-generator ofclaim 2, further comprising: a plurality of windings wound on at least aportion of the main stator; and a plurality of stator switcheselectrically coupled between selected ones of the main stator windings,each of the stator switches having at least a first position and asecond position, wherein the main stator is configured as the M-pole ACstator with the plurality of switches in the first position, and isconfigured as the N-pole DC stator with plurality of switches in thesecond position.
 4. The starter-generator of claim 3, wherein thecontrol circuit comprises: switch control circuitry operable to move theplurality of switches between at least the first position and the secondposition.
 5. The starter-generator of claim 1, further comprising: anexciter rotor rotationally mounted in the housing, the exciter rotorhaving a plurality of windings wound thereon; an exciter stator having aplurality of windings wound thereon, the exciter stator mounted withinthe housing and located at least partially around at least a portion ofthe exciter rotor; and one or more rectifier assemblies electricallycoupled in series between the exciter rotor windings and the main rotorwindings.
 6. The starter-generator of claim 5, further comprising: aplurality of main rotor windings wound on at least a portion of the mainrotor, each of the main rotor windings adapted to selectively receive DCcurrent from either the rectifier assemblies or a DC power source. 7.The starter-generator of claim 6, further comprising: at least twobrushes adapted to electrically couple to the DC power source andselectively moveable into, and out of, electrical contact with at leasta portion of the main rotor, whereby the brushes are electricallycoupled to, and decoupled from, respectively, the main rotor windings.8. The starter generator of claim 1, further comprising: a DC powersource electrically coupled in series with the main stator when the mainstator is configured as an N-pole DC stator.
 9. The starter-generator ofclaim 1, wherein M is unequal to N.
 10. The starter-generator of claim9, wherein N=(M/2).
 11. A stator, comprising: a main stator body; aplurality of stator coils wound around at least a portion of the mainbody in a configuration that allows the stator to be selectivelyconfigured as an M-pole AC stator or an N-pole DC stator, M and N eachintegers greater than one, wherein: when the stator is configured as theM-pole AC stator, each phase consists of two or more stator windingcircuits electrically coupled in parallel with one another, and when thestator is configured as the N-pole DC stator, the stator windingcircuits associated with each phase are directly connected in serieswith one another.
 12. The stator of claim 11, further comprising: aplurality of stator switches electrically coupled between selected onesof the stator coils, each of the stator switches having at least a firstposition and a second position.
 13. The stator of claim 12, wherein thestator is configured as the M-pole AC stator with the stator switches inthe first position, and is configured as the N-pole DC stator with thestator switches in the second position.
 14. The stator of claim 11,wherein M is unequal to N.
 15. The stator of claim 14, wherein N=(M/2).